1. Field of the Invention
The present invention relates to image display devices and methods of controlling the same, and in particular to an image display device using a current-driven luminescence element and a method of controlling the same.
2. Description of the Related Art
Image display devices in which organic electro-luminescence (EL) elements are used are known as image display devices with which current-driven luminescence elements are used. The organic EL display devices using organic EL elements which emit light are best suited to make thinner devices because such organic EL elements eliminate the necessity of back lights conventionally required for liquid crystal display devices. In addition, the organic EL elements do not place a limit on view angle, and thus are expected to be practically used as next-generation display devices. Further, the organic EL elements used for the organic EL display devices including luminance elements whose luminance are controlled by currents having certain values, instead of including liquid crystal cells controlled by voltages to be applied thereto.
In a usual organic EL display device, organic EL elements which serve as pixels are arranged in a matrix. An organic EL display is called a passive-matrix organic EL display, in which organic electro-luminescence elements are provided at intersections of row electrodes (scanning lines) and column electrodes (data lines) and voltages corresponding to data signals are applied to between selected row electrodes and the column electrodes to drive the organic EL elements.
On the other hand, an organic EL display device is called an active-matrix organic EL display, in which switching thin film transistors (TFTs) are provided at the intersections of scanning lines and data lines and connected with the gates of driving transistors which receive data signals, through the signal lines, when the TFTs are turned on through selected scanning lines, and causes the driving transistors to activate the organic EL elements.
Although the passive-matrix organic EL display device in which organic EL elements connected to selected row electrodes (scanning lines) emit light only until the selected row electrodes become unselected, organic EL elements in the active-matrix organic EL display device keep emitting light until they are scanned (or selected). Thus, there is no reduction in luminance even when the number of scanning lines increases. Accordingly, the active-matrix organic EL display device is driven with a low voltage, thereby consuming less power.
Patent Reference (Japanese Unexamined Patent Application Publication No. 2005-4173) discloses a circuit configuration of pixel units in an active-matrix organic EL display device.
FIG. 16 is a diagram showing a circuit configuration of a pixel unit in a conventional organic EL display device disclosed in Patent Reference. The pixel unit 500 is configured with a simple circuitry including: an organic EL element 505 having a cathode connected to a negative power source line (whose voltage value is denoted as VEE); an n-type thin film transistor (n-type TFT) 504 having a drain connected to a positive power source line (whose voltage value is denoted as VDD) and a source connected to the anode of the organic EL element 505; a capacitor element 503 which is connected to between the gate and source of the n-type TFT 504 and holds a gate voltage of the n-type TFT 504; a third switching element 509 for causing both the terminals of the organic EL element 505 to have approximately the same potential; a first switching element 501 which selectively applies a video signal from a signal line 506 to the gate of the n-type TFT 504; and a second switching element 502 for initializing the gate potential of the n-type TFT 504 into a predetermined potential. The following describes light emitting operations performed by the pixel unit 500.
First, the second switching element 502 is brought into an on state by a scanning signal supplied from the second scanning line 508. A predetermined voltage VREF supplied from a reference power source line is applied to the gate of the n-type TFT 504 so as to prevent a current from flowing into between the source and drain of the n-type TFT 504 in order to initialize the n-type TFT 504.
Next, the second switching element 502 is brought into an off state by a scanning signal supplied from the second scanning line 508 (S102).
Next, the first switching element 501 is brought into an on state by a scanning signal supplied from the first scanning line 507 to apply a signal voltage supplied from the signal line 506 to the gate of the n-type TFT 504 (S103). At this time, the gate of the third switching element 509 is connected to the first scanning line 507, and thus becomes conductive simultaneously with the first switching element 501. This makes it possible to accumulate charge corresponding to a signal voltage in the capacitor element 503 without being affected by the voltage between the terminals of the organic EL element 505. In addition, the organic EL element 505 is not supplied with a current while the third switching element 509 is conductive, and thus does not emit light.
Next, the third switching element 509 is brought into an off state by a scanning signal supplied from the first scanning line 507 to supply a signal current corresponding to the charge accumulated in the capacitor element 503 from the n-type TFT 504 to the organic EL element 505 (S104). At this time, the organic EL element 505 emits light.
The sequential operations described above enable the organic EL element 505 to emit light with a luminance corresponding to the signal voltage supplied from the signal line in a frame period.